Lotus Elan

Calculate compression ratio from compression tester result

PostPost by: JonB » Wed Sep 27, 2023 3:49 pm

Hi

Bit of a technical one, this.. Can the Twink's compression ratio be worked out (even approximately) from the readings taken from a compression tester?

For example, I measured 192, 196, 199, 210 PSI respectively in a cold crank test (albeit with the throttle closed) for cylinders 1,2,3,4 respectively. Is there a formula for calculating the compression ratio from these?

Ta muchly
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PostPost by: promotor » Wed Sep 27, 2023 7:43 pm

I'd say it would be hard to get an accurate compression ratio figure - different cams will give different readings, cam timing and even small things like how loose or tight cam/valve clearance figures are will make a difference. Piston ring sealing will also vary results so it would be an educated guess.
Last edited by promotor on Wed Sep 27, 2023 9:14 pm, edited 1 time in total.
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PostPost by: john.p.clegg » Wed Sep 27, 2023 8:55 pm

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PostPost by: JonB » Thu Sep 28, 2023 9:08 am

Thanks John, very useful. I'm figuring something like 10.5:1 ratio but I will do another test once the engine is running.. in about 100 years at this rate...
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PostPost by: Donels » Thu Sep 28, 2023 5:06 pm

That's a cracking document. Very interesting.
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PostPost by: Spyder fan » Thu Sep 28, 2023 5:53 pm

If I remember correctly the standard compression ratio for a lotus twin cam of your cars vintage is 9.5:1

No doubt those with more definite knowledge will be along shortly. 10.5:1 would be a modified/uprated engine
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PostPost by: JonB » Fri Sep 29, 2023 8:07 am

Interesting, Alan. It's an S/E engine with a D cam. I've seen these engines referred to as "Super S/E".
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